A passive infrared intrusion detector, as is known in the art, detects changes in the level of infrared rays impinging upon a passive infrared sensor which receives infrared rays from an area to be monitored through a lens device. The lens device optically divides the area into a plurality of zones from which rays can be received. The zones are separated from one another, so that when an object (i.e. a person or an object) is moved across the zones, the detector will receive rays from the object when the object is in one of the zones, and will receive rays only from the background when the object moves into the space between the zones. The result is a chopping or a flicker of infrared radiation received by the detector. The pyroelectric sensor then produces a signal in response to the chopping or flicker which can be processed to trigger an alarm.
The signal processing means to determine whether an alarm should be triggered or not, is a very important element in the detection system.
The most basic signal processing means is the use of a threshold, that is when the signal from the pyroelectric sensor has an amplitude which exceeds a preset level, the alarm is triggered. The basic threshold method is prone to lack of sensitivity and/or setting false alarms due to spurious background noise (e.g. heat emitted from sedentary objects or small animals such as mice).
Other more sophisticated signal processing means include requiring that the signal cross both a positive and a negative threshold as disclosed in U.S. Pat. No. 4,179,691, counting the number of times the signal crosses the threshold as disclosed in U.S. Pat. No. 4,764,755, and preventing a potential trigger when a signal exceeds a threshold for too little time (i.e. spikes) as disclosed in U.S. Pat. No. 4,612,442. The methods listed above are, however, still prone to lack of sensitivity or false alarms.
It is therefore an object of the present invention to provide an infrared intrusion detection system including a signal processing means, which is less prone to lack of sensitivity or false alarms.